If a paper web is processed in a folding apparatus immediately after having been imprinted, without the printing ink having had sufficient time for drying, there is the danger that, because of the contact of the paper web with the rollers of the folding apparatus, ink may be smudged or may be transferred from one web to another web because of the contact between several webs of material being processed simultaneously in the folding apparatus. Modern printing presses attain such high web speeds that the length of time between the imprinting of a web section and its arrival at the folding apparatus is of a length of only a few fractions of a second. Sufficient drying of the ink is not possible during this length of time if it is not speeded up by the provision of technical aids.
Drying devices for drying a freshly imprinted web of material are shown, for example, in DE 41 33 555 A1 or DE 44 29 891 A1.
DE 41 33 555 A1 describes a rotogravure printing press with several printing rollers for use in multi-color printing. After passing over every individual printing roller, a web of material travels over a transport path whose direction is changed by several rollers, on which transport path drying devices are arranged. Here, the course of the transport path has been selected in such a way that the first change of direction effecting rollers, over which the web of material runs after having passed through a printing gap, touch the non-printed back of the web. Only after the web has passed through the drying devices, and there is no longer a danger of smudging the ink, by contact with the change of direction roller, change of direction rollers follow, which also touch the imprinted surface of the web.
In connection with printing presses, for use in imprinting both sides of a paper web, the construction described in DE 41 33 555 A1 cannot be used. Contact of the freshly imprinted web with a change of direction roller, or with any other arbitrary surface, is to be avoided so long as the printing ink has not dried completely.
DE 44 29 891 A1 therefore uses a longitudinally extending drying oven for drying a web which is imprinted on the front and back, and through which the printed web runs in a straight line. It would be desirable to be able to conduct the web vertically upward, in the same direction in which it leaves the printing group, through the drying oven in order to prevent, in this way, contact of the not yet completely dried printed web with a change of direction roller. However, such an arrangement would require a structural height of several meters. Therefore, it would be difficult to install such a machine in a work room. To avoid this, and to be able to install the drying oven horizontally, a change of direction roller between the outlet of the printing group and the inlet of the drying over has to be accepted. Although an arrangement with a horizontally oriented drying oven described in the above-mentioned publication does not require any additional external height of the work room for its installation, it does require a considerable base area, since a length of several meters of the drying oven is required to achieve a dwell time in the drying oven which is sufficient for drying the ink on the imprinted web of material Although a portion of this base area can be used for installing roll changers for the printing group underneath the drying oven, for reducing the space requirement of such a printing installation it is necessary to be able to reduce the length of the drying oven. This requirement for length reduction occurs, to a greater extent, as a function of the higher the web speeds are in the printing installation. To assure sufficient web drying, with increasing web speeds, it is necessary, in connection with the known construction, to increase the length of the drying oven proportionally to the web speed.
DE 298 19 202 U1 discloses a dryer in which a web of material is rerouted. This dryer uses turning stations on which air is blown.
DE 100 44 676 A1 and DE 40 33 642 A1 show devices for rerouting a web of material by the use of compressed air.